1. Field of the Invention
This invention relates to chaff with transient radar absorbance and/or reflectance characteristics, having utility as an electronic warfare countermeasure useful in an absorbance mode for at least partial attenuation of radar signals, and useful in a reflectance mode as an electromagnetic detection decoy or for anti-detection masking of an offensive attack.
2. Description of the Related Art
In modern warfare, a wide variety of weapons systems are employed which operate across the electromagnetic spectrum, including radio waves, microwaves, infrared signals, ultraviolet signals, x-rays and gamma rays.
To counter such weapons systems, smoke and other obscurants have been deployed. In the past, smoke has been variously employed as a means of protection of ground-based military vehicles and personnel during conflict, to blind enemy forces, to camouflage friendly forces, and to serve as decoys to divert hostile forces away from the positions of friendly forces. With the evolution of radar guided missiles and increasing use of radar systems for battlefield surveillance and target acquisition, the obscurant medium must provide signal response in the millimeter wavelengths of the electromagnetic spectrum.
The use of "chaff", viz., strips, fibers, particles, and other discontinuous-form, metal-containing media to provide a signal response to radar, began during World War II. The first use of chaff involved metal strips about 300 millimeters long and 15 millimeters wide, which were deployed in units of about 1,000 strips. These chaff units were manually dispersed into the air from flying aircraft, to form chaff "clouds" which functioned as decoys against radars operating in the frequency range of 490-570 Megahertz.
Chaff in the form of aluminum foil strips has been widely used since World War II. More recent developments in chaff technology include the use of aluminum-coated glass filament and silver-coated nylon filament.
In use, chaff elements are formed with dimensional characteristics creating dipoles of roughly one-half the wavelength of the hostile electromagnetic system. The chaff is dispersed into a hostile radar target zone, so that the hostile radar "locks onto" the signature of the chaff dispersion. The chaff is suitably dispersed into the air from airborne aircraft, rockets or warheads, or from ground-based deployment systems.
The chaff materials which have been developed to date function effectively when deployed at moderate to high altitudes, but are generally unsatisfactory as obscuration media in proximity to the ground due to their high settling rates. Filament-type chaff composed of metal-coated fibers may theoretically be fashioned with properties superior to metal strip chaff materials, but historically the "hang time" (time aloft before final settling of the chaff to the ground) is unfortunately still too short to accommodate low altitude use of such chaff. This high settling rate is a result of large substrate diameters necessary for standard processes, typically on the order of 25 microns, as well as thick metal coatings which increase overall density. A further problem with metallized filaments is that typical metal coatings, such as aluminum, remain present and pose a continuing electrical hazard to electrical and electronic systems after the useful life of the chaff is over.
Although chaff has been highly successfully employed by the prior art, and is routinely used to decoy or divert radar guided threats away from military aircraft, successful exploitation of chaff typically requires precise synchronization of the chaff dissemination with an evasive maneuver. Thus, training is critical to the effective use of chaff for enhanced survivability against radar-based threats. Though the need for realistic training is universally accepted, environmental concerns over the effects of chaff pose a serious and growing threat to peacetime training in its use.
Considering the environmental impact of chaff material usage, in warfare as well as peacetime training applications, it is recognized that conventional chaff materials can measurably affect the electromagnetic environment long after their defense utility has passed. It also is recognized that long-term physical health risks may result from the persistence of the glass fibers which serve as the substrate for standard aluminized chaff materials.
In this respect, the persistence of aluminized glass fibers after settling to the ground and the perception of respiratory or digestive hazards of these materials has limited the open air dissemination of chaff for testing and training. Ranchers and environmentalists, particularly in the midwestern U.S., have objected to chaff because it is carried uncontrollably by the wind and settles on populated areas or on land grazed by wildlife and livestock. Additionally, the United States FAA has experienced difficulties in monitoring air traffic because of the electromagnetic noise associated with drifting chaff.
It would therefore be a substantial advance in the art to provide a chaff material which variously is characterized by a reduced settling rate and increased hang time, as compared with conventional chaff materials, overcomes the persistence of adverse electrical characteristics which is a major disadvantage of conventional chaff materials, and is of enhanced environmentally acceptability in relation to conventional chaff materials.
Accordingly, it is an object of the present invention to provide an improved chaff material which overcomes such difficulties and deficiencies of the prior art.
It is another object of the present invention to provide a chaff material having a metal component with an evanescent electromagnetic detection signature.
It is another object of the present invention to provide a chaff material whose electronic signature may be selectively adjusted so that the chaff material is transiently active for a predetermined time, consistent with its purpose and its locus of use.
It is a further object of the invention to provide a chaff material which is environmentally degradeable to relatively benign end products.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.